The following morning, she was arrested, along with her Italian boyfriend, Raffaele Sollecito, 23 and Patrick Diya Lumumba, a 37-year-old barman from the Congo. The three are being held in custody until they can be charged with murder and sexual assault.

Miss Kercher, 21, was found with her throat cut on Friday, Nov 1, in the house she shared with Knox.

Police believe she was forced to have sex at knifepoint before being killed.

Knox also wrote: "Mom, please come and take me shopping".

A source close to the investigation said the letter shone some light on her "psychological state" just a few days after her flatmate had been murdered.

Elsewhere in the two-page letter, she wrote that she "wanted to restart" and needed "to turn over a leaf" and "leave what has happened behind".

Antonella Negri, Knox's professor, said she gave the letter to the police after she read it out to Knox's fellow students.

"They seemed shocked," she said.

She said Knox was a model student.

"Students do not normally sit at the front of the class, but Amanda always did. She seemed a sweet and innocent girl. She spoke of Seattle, her city, which is actually twinned with Perugia.

"She promised to do a project on her city, to talk about its history and its attractions. However, she was unfortunately unable to finish it," she said.

She added: "I remember another characteristic: she was always punctual. At 9am she was always there before everyone else. And we knew that she worked late at Patrick's bar.

"She gave guitar lessons to a girl from Kazakhstan, and she was always punctual about that too."

Knox's behaviour after the discovery of Miss Kercher's body is being closely examined by investigators.